Support for BYO Custom DNS with Private Clusters

[raykao]

Currently AKS Private Clusters are not compatible with BYO Custom DNS servers as there is a chicken/egg scenario.

AKS Private Clusters will also create/deploy a Private DNS Zone to enable Private Link to the AKS CP. When using Custom DNS, forward lookups need to forward to Azure DNS (168.63.129.16) or be specific with conditional forwarding.

An example implementation with Bind9 or equivalent would be helpful.